The present invention relates to a sample plate for an ion source, an ion source, a mass spectrometer, a method of ionising a sample and a method of mass spectrometry.
Electrospray ionisation (“ESI”) is known wherein charging of analyte ions occurs within the liquid phase prior to the spraying of charged droplets towards an inlet of a mass spectrometer. Evaporation of the droplets leads to the formation of multiply charged gas phase ions. The charge distribution approximately reflects the charges that were electrophoretically generated in the liquid phase.
Another method of ionisation is known and is referred to as Sonicspray ionisation (“SSI”) which uses a high pressure nebuliser to produce droplets without there being a potential drop between the ion source and the inlet to the mass spectrometer. According to this approach both singly and multiply charged ions are generated either with or without a voltage being applied to the liquid phase. However, significantly more ions are generated when a voltage is applied to the sample liquid. The charge is believed to come from the statistical imbalance and distribution of charges prior to droplet disruption due to shear stress.
Matrix Assisted Laser Desorption Ionisation (“MALDI”) is another known ionisation process and uses a solid crystalline matrix with analyte embedded within it. In the dried solid crystalline phase there is little, if any, charge mobility within the matrix. Singly charged species are typically dominant after analyte protonation. Multiply charged ions have been observed at low levels but the process of generating multiply charged ions by MALDI has not been fully understood or commercially exploited.
WO 2012/058248 and US 2012/0085903 disclose a method of ionisation known as Laserspray ionisation (“LSI”). Laserspray ionisation, like MALDI, uses a solid matrix. Although singly charged species are often generated, multiply charged ions are more prevalent when the desorbed particles are caused to collide with a heated vacuum transfer tube. Droplets then attain an energy component capable of shearing the charge separated particles (probably droplets) in a similar manner to Sonicspray. Charge mobility within the molten matrix droplet via a double layer formation effect may contribute to the charge states.
Matrix Assisted Inlet Ionization (“MAII”) is similar to Laserspray but does not use a laser. Instead, heat is used to vaporize the matrix into vacuum or a mechanical disturbance causes the matrix to enter a heated transfer tube. Multiply charged ions can be generated. Liquid, crystal or particle fracturing has been given as a possible explanation for generating multiply charged ions in a similar manner to Laserspray.
Solvent Assisted Inlet Ionization (“SAII”) is another ionisation method which produces multiply charged ions when droplets interact (and receive energy) from heated surfaces. The ion signal is orders of magnitude higher when the liquid has been pre-charged using a voltage (like Sonicspray). The charges within the liquid droplets are mobile and become stratified and when shattered by collisions produce highly charged analyte ions.
US 2003/0066957 (Andersson) discloses a microfluidic device in the form of a disc. FIG. 4e shows a cross-sectional view of an Energy Desorption Ionisation (“EDI”) area.
U.S. Pat. No. 5,260,571 (Cottrell) discloses an arrangement with reference to FIG. 2 wherein a mixture of proteins is separated electrophorectically in a slab 21 of polyacrylamide gel. The gel 21 is placed in a blotting tank 22 having a bottom electrode 24. One or more targets 25 precoated with a substrate material are placed face down on the upper surface of the gel 21. A potential difference of a few tens of volts is applied between the bottom electrode 24 and the conductive targets 25 which induces proteins to migrate from the gel 21 towards the targets 25 where they are bound by the substrate material.
US 2010/0323917 (Vertes) discloses the production and use of semiconducting nanopost arrays made by nanofabrication.
US 2004/0094705 (Wood) discloses a microstructured polymeric substrate.
US 2008/0245961 (Choi) discloses a nanowire-assisted method for mass spectrometric analysis of a specimen.
US 2008/0156983 (Fourrier) discloses an integrated system for microfluidic analysis. FIGS. 1-4 disclose an arrangement for moving drops on a track. With reference to FIG. 5 by successively applying potential differences between electrodes 2a-2h and line 5 it is possible to move a drop 14 and to immobilise the drop on a pad 12a-12f. 
GB 2306644 (Apffel) discloses a liquid handling system for a MALDI-Time of Flight mass spectrometer.
It is desired to provide an improved ionisation method.